1. Field of the Invention
The invention relates to an optical amplifier arrangement for amplifying ultra-short pulsed laser radiation comprising a mode-locked laser and two or more optical amplifiers arranged downstream of the laser in the propagation direction of the laser radiation.
2. Description of the Related Art
Optical amplifier arrangements comprising a laser and downstream arranged optical amplifiers are known in large number from prior art. These arrangements are required to amplify the output radiation from pulsed lasers so as to ensure sufficient pulse energy in order to meet tasks in materials processing in medicine or science.
The output energy and the pulse peak rate of ultra-short pulse amplifiers known in prior art are limited due to various physical effects or technical features of the amplifier arrangement. For example, a high peak intensity of the short laser pulses leads to non-linear effects in the amplifier medium which deteriorate pulse quality and thus limit the maximum pulse peak rate in the amplifier. Likewise, the laser material destruction threshold and/or the maximally possible thermal load due to absorption confine the achievable amplifier output rate.
Ultra-short pulse amplifiers known in prior art include chirped pulse amplification amplifiers (CPA amplifiers). With these amplifiers, pulses to be amplified are at first temporally stretched out by means of a pulse stretcher so that the pulse peak rate is diminished. Pulses stretched out this way are subsequently amplified by means of one or more amplifiers and temporally re-compressed within a pulse compressor. Hereby it is possible to achieve a high amplification factor whilst avoiding non-linear effects.
Furthermore it is known in prior art evading a degradation in pulse quality due to non-linear effects by applying the method of pulse forming. With this method, the phase, amplitude or other parameters of the laser pulse are varied by the aid of suitable optical elements, e.g. optical modulators.
When using fiber-optical amplifiers, so-called LMA fibers are often utilized. In contrast with conventional single mode fibers, these large mode area fibers have an enhanced core diameter in which, however, only a few modes are capable of propagating. Thus, on account of the larger radiation area, a variation of the mode field diameter enables a corresponding increase in the possible pulse peak rate.
Furthermore known in prior art is the use of external passive resonators in which the pulses are coherently superimposed, thus causing a rise in the intra-cavity rate, in order to amplify ultra-short laser pulses
It is also known combining several lasers with different frequencies incoherently to achieve high-energetic laser radiation. Scaling the output rate in cw mode or in pulsed mode with pulse lengths in the nanosecond range is thus made possible.
In prior art, it is also known to combine the output rate of several cw lasers coherently, with it being possible to scale the output rate as demanded. Coupling is accomplished by applying the technique of a so-called self-injection seeding and/or active stabilization.
Besides, a coherent superposition of laser pulses may also be realized with spectrally separated single spectra. Thus it is made possible to generate pulses having pulse lengths substantially reduced as compared with output pulses.
The approaches outlined hereinabove and geared to generate high-efficient ultra-short laser pulses can achieve good results depending on a given application situation.